KB5078127

Microsoft has released an out-of-band cumulative update KB5078127 (build 26100.7628) for Windows 11 versions 22H2 and 24H2 (2024 Update). It fixes a critical issue causing application freezes and errors when working with files in cloud storage — this failure appeared after the January patch. The update is intended for Windows 11 24H2 (Windows 11 2024 Update) and Windows 11 LTSC 2024 on x64 (amd64) and ARM64 processors, and is distributed through Windows Update and the Microsoft Update Catalog.

• Fix for application freezes. After installing the January updates from January 13, 2026, some programs might stop responding or throw errors when opening or saving files in cloud storage such as OneDrive or Dropbox. Update KB5078127 fully resolves this issue.
• Fix for Outlook crashes with PST files. In certain configurations of Outlook, when PST files were stored in OneDrive, the application could freeze and fail to restart without ending the process or rebooting the system. Users could also encounter sent messages disappearing or previously received messages being downloaded again — this is now fixed.
• Includes all previous improvements. The update is cumulative and includes all security fixes and improvements from the January security update KB5074109 from January 13, 2026, as well as from the out-of-band update KB5077744 from January 17, 2026.

No known issues have been reported so far, as the update is specifically released to quickly fix the critical cloud storage failure. Microsoft recommends that all users who have already applied the January updates KB5074109 or KB5077744 and experienced the described symptoms install it.

Update KB5078127 is distributed via Windows Update for devices that already have updates KB5074109 or KB5077744 installed. To install it, go to Settings > Windows Update and click Check for updates. The package is also available for manual download from the Microsoft Update Catalog. It is recommended to save all open documents and close applications that work with cloud files before installation.

How the new features work:

Fix for application freezes when working with cloud storage: The file synchronization mechanism via filter drivers (e.g. cldflt.sys) could block CreateFile/WriteFile calls during lock conflicts. The update changes async callback timings and properly handles the ERROR_CLOUD_FILE_SYNC_IN_PROGRESS error code, eliminating freezes.

Fix for Outlook crashes when working with PST files in OneDrive: Outlook uses legacy MAPI calls that interacted poorly with on-demand hydration of cloud files. The patch modifies cldflt.sys driver behavior, forcing the entire PST file to be cached locally before mounting, avoiding re-download of already synchronized blocks.

Cumulative nature of the update: KB5078127 fully includes previous fixes. During installation, the system verifies the component manifest via DISM, applying all changes from KB5074109 (security) and KB5077744 (out-of-band). Windows Update returns only delta packages relative to the last installed cumulative update.

The last 10 Windows updates: